Background:
https://github.com/heycam/webidl/issues/362

Change for WebIDL:
https://github.com/heycam/webidl/pull/364
https://github.com/w3c/web-platform-tests/pull/5966

Change for Geometry APIs (for WebKitCSSMatrix):
https://github.com/w3c/fxtf-drafts/pull/167
https://github.com/w3c/web-platform-tests/pull/5991

(The css/geometry-1/interfaces.html test won't pass completely until https://bugs.chromium.org/p/chromium/issues/detail?id=709001 is fixed, or if the SVGMatrix alias is removed from the spec and test IDL.)

The bindings should be exactly equivalent to the current way of doing aliases in Chromium (cf. webkitURL).